Vaccinia virus is used to vaccinate persons to prevent disease with smallpox. Compared with other live virus vaccines, the smallpox vaccine is associated with frequent side effects including fever, a sore or swollen arm, headache, and fatigue. Other more serious side effects have also been reported including development of multiple lesions outside the vaccination site (generalized vaccinia), inflammation of the brain (postvaccinia encephalitis), severe infection of the skin in persons with a history of eczema (eczema vaccinatum), and disseminated vaccinia virus infection in immunocompromised persons (progressive vaccinia). [unreadable] [unreadable] Relatively few studies of vaccine-induced immune responses have been done in the modern era using contemporary techniques. Cytokines are proteins that are produced by white blood cells and secreted into the bloodstream that are important for trafficking of white blood cells to trigger inflammation. We hypothesize that induction of cytokines is associated with some of the side effects of smallpox vaccination. Identification of specific cytokines induced after vaccination may help to explain the pathogenesis of certain side effects associated with smallpox vaccination and might suggest new ways to modify some of these side effects.[unreadable] [unreadable] Studies in the 1930s and 1950s using smallpox vaccine strains that were likely less attenuated than the current New York City Board of Health (NYBOH) strain reported viremia (i.e. positive culture for virus in the blood) between the third and tenth day after vaccination. A study in 1964 suggested that viremia is rare, except in immunocompromised persons or in those with multiple cutaneous lesions. Studies performed in 2004 and in 2006 using the NYBOH strain of smallpox vaccine reported no evidence of virus in the blood from vaccinees by the polymerase chain reaction (PCR) or culture. However a second study in 2006 detected smallpox vaccine virus in the blood by PCR in 5 of 77 vaccinees, but virus cultures were not performed. This latter study led the authors to conclude that the duration of deferral for blood donation from smallpox vaccine recipients should be extended. We tested blood from 27 smallpox vaccine recipients and found that 4 of 202 blood samples (from 3 of 27 vaccine recipients) were positive for the smallpox vaccine virus in the blood by PCR, but none were positive by culture. The lack of finding infectious smallpox vaccine virus in the blood suggests to us that the current guidelines for deferral of blood donation from smallpox vaccine recipients are appropriate.[unreadable] [unreadable] Studies in the 1930s and 1950s using smallpox vaccines that likely were more virulent than the NYBOH strain reported detection of infectious smallpox vaccine virus in throat swabs in some persons 3 to 5 days after vaccination. A study in 2005 using the current smallpox vaccine strain found no evidence of smallpox vaccine virus by PCR or culture in the throat of smallpox vaccine recipients. In contrast, a study in 2006 detected smallpox vaccine virus DNA in throat swabs in 11 of 77 vaccinees by PCR, but cultures were not performed. We tested throat swabs from 28 smallpox vaccine recipients and found that 0 of 213 throat swabs were positive by culture and 0 of 190 throat swabs were positive by PCR. This study suggests that few, if any, healthy smallpox vaccine recipients shed the virus from their throat.